1. Field of the Invention.
This invention relates to an amplifier circuit for use with, for example, a pH sensor and which needs only three wires for connection to a remote energization and monitoring circuit. The circuit includes apparatus to diagnose sensor problems as, for example, whether or not the electrodes are coated, cracked or broken.
2. Description of the Prior Art.
In many sensing situations, for example pH sensing, it is customary to utilize an amplifier located in the vicinity of a pH sensing probe to act as a preamplifier or impedance converter to amplify or buffer the signal from the probe for transmission to a more remote monitoring and energization apparatus. Prior art circuits have used four wires for connection between the remote energization circuit and the preamplifier associated with the pH probe, i.e., (1) a conductor for receiving the positive supply, (2) a conductor for receiving the negative supply, (3) a common conductor for use as a reference, and (4) a conductor to transmit the signal output from the pH sensor. Three-wire preamplifier design is also known, but such design requires extensive bootstrap and bias circuitry for support. Not only have such systems involved unnecessary cost and complexity, the additional biasing circuitry requires more room and is less easily fitted into an electrochemical probe than desired.
It is also desirable to be able to diagnose a broken or cracked electrode or one that is coated without having to utilize additional circuitry. It is furthermore desirable to keep the level of current in the circuit as low as possible to minimize errors due to voltage drops in the power supply lines, and the system should preferably be able to operate at very low energization levels in order to conserve power and can be used in intrinsically safe applications.